1. Field of the Invention
The present invention relates to an aligning device for a projection exposure apparatus for semiconductor device manufacture, and more particularly to a focusing device for alignment in the axial direction of the projection lens.
2. Related Background Art
So-called stepper, or a reduction projection exposure apparatus of step-and-repeat type employed principally in the photolithographic process for semiconductor device manufacture, exposes a photosensitive substrate (hereinafter called wafer) to a circuit pattern formed on a mask or a reticle (hereinafter collectively called reticle) through a projection lens. Unless the projected image of the circuit pattern of the reticle is correctly focused on the wafer, the resulting image becomes blurred and cannot provide sufficient resolution. Such stepper employs a projection lens of a large numerical aperture (N. A.) for obtaining a high resolving power. The numerical aperture of the projection lens has been increased, corresponding to the minimum line width of the submicron circuit pattern, so that the practical depth of focus has become very small.
Also a prolonged exposure operation results in a temperature rise in the projection lens due to the absorption of irradiation energy therein, and the image plane of the projection lens may vary in the axial direction due to the temperature change of said projection lens, or the energy accumulation therein. Thus insufficient resolution is encountered on the wafer unless the projected image of the circuit pattern of the reticle is exactly focused on the wafer.
In order to obtain a semiconductor integrated circuit of desired characteristics, it is essential to exactly align the focal plane of the projection lens with the surface of the wafer. For achieving such focusing performance, there is disclosed a device for example in the U.S. Pat. No. 4,650,983. Such device is provided with so-called through-the-lens (TTL) optical system for detecting a first mark on the reticle and detecting a second mark on the wafer through the projection lens, and is so designed as to effect the focusing for said first mark by adjusting said first mark detecting optical system, and to effect the focusing for said second mark by axially varying the distance between the wafer and the projection lens. Consequently the reticle is always maintained conjugate with the wafer with respect to the projection lens, and the projected image of the circuit pattern of the reticle is always formed in a bast focus state on the wafer.
However, in such device, in the focusing operation by varying the distance between the wafer and the projection lens in the axial direction, it is necessary to move the wafer stepwise by an amount .DELTA.n and to detect the contrast of the image of the second mark at each position of the wafer by means of the TTL optical system. Similar measurements are also required in the focusing of the TTL optical system with the reticle. Consequently the throughput is deteriorated because the focusing requires time.
Also in such device, the first mark on the reticle is associated with the pattern area, corresponding to the peripheral area in the exposure field of the projection lens, so that the state of focus is always detected in such peripheral part of the exposure field. Consequently the focus state at the central part of the exposure field cannot be detected, and the image plane curvature of the projection lens cannot be measured. For this reason, a test exposure has to be made in order to measure the image plane curvature. Besides, the focus position is usually detected at the position of a rectangular mark extended in the sagittal direction (S-direction) on the reticle, but thus detected focus position in the sagittal direction is offset from the focus position detected from a rectangular mark extended in the meridional direction (M-direction), due to the astigmatism of the projection lens. This phenomenon deteriorates the precision of detection of the focus position, and the limit of resolution of the projection lens considered in the focus position in the sagittal and meridional directions is reduced. Furthermore, such device is unable to control the surface position of the wafer in response to the change of the image plane resulting from the accumulation of the irradiation energy in the projection lens, and the imaging performance at exposure is deteriorated due to such defocus, image plane curvature or image plane inclination.
In the exposure apparatus disclosed in the above-mentioned U.S. Pat. No. 4,650,983, the second mark on the wafer is illuminated through the projection lens, and the reflected light is detected again through the projection lens. Thus the passing of the illuminating light through the projection lens twice not only causes accumulation of the irradiation energy in the projection lens but also induces a significant loss of the light intensity, thereby eventually deteriorating the focusing precision. In order to prevent such drawback, the Japanese Laid-open Patent Sho No. 63-70104, corresponding to the U.S. Pat. Application Ser. No. 94,448 filed Sept. 9, 1987, discloses an optical device in which the second mark is illuminated from the rear side, and the light transmitted by said second mark is detected through the projection lens. In such known device, the pupil of the projection lens is divided into two portions, and the transmitted light is detected by shielding one of thus divided two portions. Consequently the detection of the best focus position requires a long time, as the shielded portion has to be switched at each detection.